Research on offspring sex ratios and parental condition indicate that parents in better condition may benefit from biasing their offspring toward sons. While measures of phenotypic quality and offspring sex ratio have been examined in other animals, the current study was designed to look at this relationship within human families.
I investigated the relationship between fluctuating asymmetry and facial masculinity, both possible measures of phenotype, and offspring sex ratio within families.
The results were mixed. The fluctuating asymmetry of one sibling covaried negatively with the overall offspring sex ratio within their family. The significant negative correlation between the two provides partial support for adaptive biasing in offspring sex ratio. However, no relationship was detected between male facial masculinity, female facial masculinity, and sex ratio within families.